1. Field of the Invention
The present invention relates to a piezoelectric device. More particularly, the invention concerns a piezoelectric device that in case of a piezoelectric resonator suppresses the occurrence of spurious waves and in case of a two-pole monolithic filter makes the bandwidth larger and suppresses the spurious waves.
2. Description of the Related Art
Piezoelectric devices have been being used in many communication apparatus as electronic devices each of that enables obtaining excellent frequency/temperature characteristics over a wide range of frequency from several tens of kHz to several hundreds of MHz and is small in size and also is solid.
FIGS. 5(a) and 5(b) are a plan view and a sectional view taken along a line Qxe2x80x94Q, which illustrate the construction of an AT cut crystal resonator. Substantially at the centers of the both surfaces of an AT cut crystal substrate 31, (hereinafter referred to as xe2x80x9ca substratexe2x80x9d) there are disposed mutually opposing electrodes 32a and 32b. From these electrodes 32a and 32b there are extended toward the edges of the substrate 31 lead electrodes 33a and 33b.An AT cut crystal resonator element is thereby formed. This crystal resonator element is accommodated within a package (not illustrated), and the lead electrodes 33a and 33b are connected to the terminal electrodes of the package, respectively, using electrically conductive adhesive, etc. A crystal resonator is thereby formed.
Applying a high-frequency voltage across the lead electrodes 33a and 33b of the AT cut crystal resonator illustrated in FIGS. 5(a) and 5(b), two kinds of thickness vibrations are excited. One is a thickness twist mode of vibration that propagates in the Zxe2x80x2-axial direction and the other is a thickness shear mode of vibration that propagates in the X-axial direction. However, in general, these two kinds of modes of vibrations are called xe2x80x9cthe thickness shear modexe2x80x9d of vibration, generically.
While various methods of analyses have been used as those for analyzing the thickness shear mode of oscillation, it is well known that an energy trapping theory has been widely used on account of its brevity.
Assume that various parameters of the crystal resonator be set as illustrated in FIG. 5(c). Namely, assume that H represents the thickness of the substrate; fs represents the cut-off frequency of the substrate; L represents the size of the electrode; and fe represents the cut-off frequency of the electrode part. Then,the resonance frequency fr of the crystal resonator is located between the cut-off frequencies fe and fs as illustrated in FIG. 5(d). According to the energy trapping theory, the energy trapping coefficient P is defined as in the following equation.
P=(xcfx80{square root over ( )}2) xcexcL/H{square root over ( )}xcex94xe2x80x83xe2x80x83(1)
Also, except for the constant (xcfx80{square root over ( )}2), the energy trapping coefficient P is sometimes defined as in the following equation.
Pxe2x80x2=xcexcL/H{square root over ( )}xcex94xe2x80x83xe2x80x83(2)
where xcexc represents the constant that is primarily determined from the elastic constants of the substrate. Accordingly, the mass loading is defined as in the following equation.
xcex94=(fsxe2x88x92fe)fsxe2x80x83xe2x80x83(3)
The energy trapping coefficient is an important parameter for determining up to which vibration mode should be set under the category of the xe2x80x9ctrapped modexe2x80x9d.
For example, the energy trapping coefficient Pxe2x80x2 for which only a primary symmetric mode of the fundamental wave alone is set as the trapped mode is theoretically 2.17 and 2.75, respectively, for the thickness twist mode and for the thickness shear mode. However, actually, the energy trapping coefficient Pxe2x80x2 is not as theoretically. Correcting each of these values experimentally so that the amount of energy confined may become the largest, it is well known that these values should be corrected, respectively, to values of 2.4 and 2.8.
FIGS. 6(a) and 6(b) are a plan view and a sectional view taken along a line Qxe2x80x94Q, which illustrate a two-pole monolithic filter (hereinafter referred to as xe2x80x9ca two-pole monolithic filterxe2x80x9d). On one surface of a substrate 41 there are disposed electrodes 42 and 43 closely to each other, and, an electrode 44 is disposed on the other surface thereof in such a way as to oppose the electrodes 42 and 43. From the electrodes 42, 43, and 44 there are extended toward the edges of the substrate 41 lead electrodes 45, 46, and 47, thereby constructing a two-pole monolithic filter.
Applying a high-frequency voltage to the lead electrodes 45 and 47, as well known, a primary symmetrical mode of and a primary anti-symmetric mode are strongly excited in the electrodes 42, 43, and 44. Utilizing these two modes of oscillation waves, a two-pole monolithic filter is constructed.
Assume that fs represents the cut-off frequency of the substrate 41; and fe represents the cut-off frequency that prevails when having adhered the electrodes 42, 43, and 44 to the substrate 41. Then, the frequencies f1 and f2 of the excited symmetrical primary mode and primary anti-symmetric mode of oscillation waves become spectrum as illustrated in FIG. 6(c). Resultantly, the frequency bandwidth twice as large as that obtained as the difference between the frequencies f1 and f2 becomes a frequency bandwidth of the two-pole monolithic filter.
However, when attempting to design an oscillation device having a high frequency band of 200 MHz as the one for use in a crystal resonator or two-pole monolithic filter, even if using as the electrode materials an aluminum alloy that is light in mass, it is necessary to set the size of the electrodes to be very small in order to satisfy the above-described energy trapping coefficient. As a result, in case of a crystal resonator, there was the problem that the equivalent resistance was excessively high while, in case of a two-pole monolithic filter, there was the problem that the impedance was excessively high. Furthermore, at the time of the manufacture, because the electrode size is excessively small, there was also the problem that mask alignment was very difficult to make.
In order to solve these problems, an attempt has been made to use an entire-surface electrode as the electrode for use on one surface of a high-frequency crystal resonator or high-frequency two-polermonolithic filter. Through making this attempt, a device that has been arranged for mass loading not to contribute to the energy trapping has ever been put to practical use. However, when, for example, setting the electrode configuration on one side of a 200-MHz frequency-band two-pole monolithic filter of the fundamental-wave to be 0.15 mmxc3x970.25 mm, the energy trapping coefficient becomes excessively large. As a result of this, the problem that an inharmonic mode of oscillation waves occurs still remains unsolved.
Also, FIGS. 7(a) and 7(b) are a plan view and a sectional view taken along a line Qxe2x80x94Q, both illustrating the construction of a monolithic crystal filter that is disclosed in Japanese Patent Application Laid-Open No. Hei10-32459. This publication describes a two-pole monolithic filter comprising a substrate 51 and electrodes 52, 53, and 54. It describes that the entire remaining portion of the substrate 51 has disposed thereon electrodes for a suppression 55b, 56a, and 56b in such a way as for these electrodes to be kept at a distance from those electrodes 52, 53, and 54 of the two-pole monolithic filter. And it describes thereby suppressing the occurrence of a higher harmonic mode of oscillation waves such as unnecessary flexible vibration, contour vibration, etc., whereby excellent pass-band characteristics have been obtained.
However, according to the width of the gap between the electrodes 52, 53, and 54 of the two-pole monolithic filter and the electrodes for a suppression 55a, 55b, 56a, and 56b disposed around these electrodes 52, 53, and 54, the respective thickness-values of the electrodes 55a, 55b, 56a, and 56b, etc., it is impossible to sufficiently confine the displacements of the two vibration modes, constituting the two-pole monolithic filter, into the region defined by the electrodes 52, 53, and 54. Resultantly, the oscillatory waves in those two vibration modes are leaked into the environmental electrodes for a suppression 55a, 55b, 56a, and 56b, with the result that there newly arises the problem that the insertion loss of the two-pole monolithic filter becomes deteriorated.
The present invention has been made in order to solve the above-described problems and has an object to provide a high-frequency piezoelectric device that, regarding a desired vibration mode of oscillation waves, enables suppressing the occurrence of an inharmonic mode of waves while keeping the energy trapping coefficient to be at a satisfactory value.
To attain the above object, according to the first aspect of the invention, there is provided a piezoelectric resonator, the piezoelectric resonator having two electrodes disposed on its piezoelectric substrate, one electrode being disposed on an obverse surface of the piezoelectric substrate and the other being disposed on a reverse surface thereof in such a way that these two electrodes are opposed to each other, wherein comb-shaped electrodes each consisting of electrode fingers and spaces are disposed around the electrode located at least on one surface of the piezoelectric substrate at prescribed space intervals between this electrode and each of the, comb-shaped electrodes.
According to the second aspect of the invention, there is provided a two-polemonolithic filter, the two-polemonolithic filter having a piezoelectric substrate that has disposed on one surface thereof a pair of electrodes close to each other and has disposed on the other surface an electrode opposed to these paired electrodes, wherein comb-shaped electrodes are disposed around the electrode located at least on one surface of the piezoelectric substrate at prescribed space intervals between this electrode and each of the comb-shaped electrodes.
According to the third aspect of the invention, there is provided a two-pole monolithic filter as described in the second aspect, wherein a comb-shaped electrode having a plurality of electrode fingers is disposed between the paired electrodes close to each other.
According to the fourth aspect of the invention, there is provided a two-polemonolithic filter, the two-polemonolithic filter having a piezoelectric substrate that has disposed on one surface thereof a pair of electrodes close to each other and has disposed on the other surface an electrode opposed to these paired electrodes, wherein an electrode having a number of holes formed therein is disposed around the paired electrodes situated at least on the one surface of the piezoelectric substrate and at prescribed space intervals between these paired electrodes and that electrode.